The present inventors have developed and have introduced a double-biprism electron interferometer. See, e.g., Japanese Patent Application No. 2004-004156 and Japanese Patent Application No. 2004-102530 In such double-biprism electron interferometer, two electron biprisms are arranged in order of the traveling direction of an electron beam on an optical axis in such a manner that the upper-stage biprism is located on an image plane of a specimen observed and that the lower-stage biprism is located in the shadow area of the upper-stage biprism and potentials applied to their respective filament electrodes are changed so that an overlap area (corresponding to an interference area width W) and an overlap angle (corresponding to an interference fringe spacing s) of two electron waves (e.g., an object wave and a reference wave) can be changed arbitrarily. The upper-stage electron biprism is located on the image plane of the specimen, making it possible to prevent generation of Fresnel fringes superimposed on an interference area such as a hologram, which cannot be removed, in principle, in an electron interferometer using one electron biprism (see, for instance, Japanese Patent Application Laid-open Publication No. 2002-117800). Non-Patent Documents authored by one or more inventors include “Double-Biprism Electron Interferometory”. Ken Harada. Tetsuya Akashi. Yoshihiko Togawa, Tsuyoshi Matsuda and Akira Tonomura. Applied Physics Letter: Vol. 84. No. 17, (2004) pp. 3229-3231: and “High-Resolution Observation by Double-Biprism Electron. Holography”, Ken Harada, Tsuyoshi, Matsuda, Tetsuya Akashi, Yoshihiko Togawa and Akira Tonomura, Journal of Applied Physics: Vol. 96, No. 11, (2004) pp. 6097-6102.
There is an interferometer using a charged particle beam such as an electron or an ion or an optical interferometer using light.
A double-biprism interferometer has the same optical system as the one-stage electron biprism in terms of the one-dimensional shape of an electron hologram formed by filament electrodes, the direction of the interference area and the azimuth of the interference fringes. In other words, the longitudinal direction of the interference area is determined corresponding to the direction of the filament electrodes, and the azimuth of the interference fringes only coincides with and is in parallel with the longitudinal direction of the interference area.
When a specimen in a shape extending in one direction like a carbon nanotube is observed, the specimen (tube) and interference fringes need to be angled. It is difficult to observe the specimen in the longitudinal direction. Only a part of its elongated shape is observed by means of electron holography (for example, J. Cumings et al., PRL 88, (2002) 056804)). At present, to observe the specimen in the longitudinal direction, some divided holograms of the specimen are recorded and reproduced for synthesis, or a hologram of a wide interference area is recorded without sufficient coherence of electron beams and noise by deteriorated coherence is compensated by image processing.
In addition to independent control of two parameters of the interference area width W and the interference fringe spacing s, an interferometer which can easily observe a specimen in the longitudinal direction by a simple operation is required.